Maintaining Minimum Light Levels with LEDs Results in More Energy-efficient Growth Stimulation of Begonia

Supplemental lighting is often needed for the production of high quality bedding plants for spring sales. However, the energy costs associated with lighting can be high. To reduce the electricity costs, we developed a system that automatically controls the light output from LED grow lights to supplement the available sunlight as needed. The system provides just enough light from LEDs to assure that the total amount of light, from sunlight plus LEDs, never drops below a user-adjustable threshold. To test crop responses to different lighting treatments, we grew begonia (Begonia x tuberhybrida pendula) ‘Illumination Apricot’ with five different lighting treatments: sunlight only, 14 hr/day of supplemental light from a red and blue LED light bar, and three treatments where the photosynthetic photon flux (PPF) was maintained at a minimum level of 50, 125, or 200 µmol·m^-2·s^-1 for 14 hours/day. Compared to 14 hours/day of constant supplemental lighting, the treatments with minimum PPF levels of 50, 125, and 200 µmol·m^-2·s^-1 reduced electricity used for lighting by 93, 76, and 62%, respectively. Shoot dry weight was increased by 30, 98, 132, and 166% compared to plants not receiving supplemental light in the 50, 125, or 200 µmol·m^-2·s^-1 and 14 hr/day treatments, respectively. Providing constant supplemental light for 14 hr/day resulted in the least efficient growth stimulation, as measured by the increase in growth divided by the power used by LEDs.  These results indicate that adjusting the amount of supplemental light based on how much sun light is present can reduce electricity costs, while achieving the growth-promoting benefits of supplemental lighting in a more cost- and energy-efficient way.